$ \left(\dfrac{5}{4}\right)^{-3}$
$= \left(\dfrac{4}{5}\right)^{3}$ $= \left(\dfrac{4}{5}\right)\cdot\left(\dfrac{4}{5}\right)\cdot \left(\dfrac{4}{5}\right)$ $= \dfrac{16}{25}\cdot\left(\dfrac{4}{5}\right)$ $= \dfrac{64}{125}$